The long-term goal of this study is to understand the molecular and genetic mechanisms of action of the Drosophila JAK/STAT pathway. In mammals, this pathway is critical for hematopoiesis and immune responses, and deregulation of the signaling activity can cause immune disease and leukemia in humans. The fruit fly Drosophila melanogaster has emerged as a genetic paradigm for systematically dissecting the components in this pathway and studying their roles in vivo. Three specific aims are proposed for investigation. In specific aim 1, deletion mutants, and loss of function and gain of function mutations in the Drosophila JAK kinase Hopscotch will be used to test the hypothesis that both positive and negative regulatory domains affect the kinase activity of Hop; activation-specific antibodies will be used to examine where and when this pathway is activated in vivo. In specific aim 2, a genome-wide genetic screen has been conducted, from which several candidate suppressor deficiencies have been isolated. The responsive suppressor genes will be identified using mutant alleles in the deficiency region; the roles of these suppressor genes will be characterized and the mechanism of interaction will be studied. In specific aim 3, target genes of the Hop/Stat92E pathway will be identified using the serial analyses of gene expression (SAGE) technique, and their functional roles studied. These studies should provide important insight into the regulation of the Hop/Stat92E pathway in Drosophila development and may also help to understand oncogenesis caused by hyperactivation of the JAK/STAT pathway in humans.